Our group employed an assay of the kinetics of ex vivo radiation-induced γ-H2AX foci and co-localization of these foci with 53BP1, to compare the cellular radiosensitivity in peripheral blood lymphocytes of former radiotherapy patients who developed abnormally severe late radiation toxicity with corresponding control patients matched for age, gender, tumor type, radiation dose, and radiotherapy duration [129]. The gene discussed is H2AX; the disease is neoplasm.